Generally, disk brake pads are formed by mixing a fiber material such as organic fiber, inorganic fiber, or metal fiber, with a powdered material such, as a friction control agent or filler, and a binding agent such as a binder resin like phenol resin; and then thermoforming the mixed material composition thereof.
When this type of disk brake pad is used, a friction surface of the disk brake pad is pushed against a rotating disk rotor, whereby rotation of the disk rotor is controlled by friction action of the friction surface. Accordingly, a braking action is achieved.
FIGS. 9A and 9B shows the configuration of a conventional disk brake pad 900; FIG. 9A shows a schematic plan view of the disk brake pad 900 when viewed from the side of a friction surface 10 thereof; and FIG. 9B shows a side view of the disk brake pad 900 as viewed from the bottom of FIG. 9A.
As can be seen from FIGS. 9A and 9B, the disk brake pad 900 is formed as an integral unit with a back plate 200, and is connected to the back plate 200 by a face that is opposite a friction surface 10. Note that, the arrow Y1 indicates a rotation direction of a disk rotor, not shown, namely, a circumferential direction of the disk rotor.
In FIG. 9A, the long-dash dot line K indicates a central axis of the friction surface 10 that extends in radial direction of the disk rotor. Further, the arrow Y2 that is marked at the lower end of the long-dash dot line K indicates a direction toward a center of rotation of the disk rotor.
Hereinafter, the direction indicated by the arrow Y1 will be referred to as the “disk rotor circumferential direction Y1”. Further, the long-dash dot line K will be simply referred to as the “central axis K1” of the friction surface 10, and the direction indicated by the arrow Y2 will be referred to as the “disk rotor rotation center direction Y2”.
Conventionally, in order to improve brake noise performance of disk brakes, namely, to reduce brake noise (hereinafter referred to as “improved brake noise performance), the disk brake pad 900 with the configuration shown in FIGS. 9A and 9B, which allows pad surface contact pressure to be regulated, has been proposed. Such a disk brake pad is disclosed, for example, in Japanese Examined Utility Model Publication H7-23650 and Japanese Utility Model No. 2589510. With this disk brake pad 900, respective chamfered portions J10 are provided at an end portion of the disk brake pad 900 at an incoming disk-rotor-rotation side thereof (namely, a leading side), and an end portion of the disk brake pad 900 at the outgoing disk-rotor-rotation side thereof (namely, a trailing side).
Since the chamfered portions J10 are formed as described, edges 11 and 12 are respectively formed at the boundary edge of the friction surface 10 and the chamfered portions J10. These edges 11 and 12 are respective edges of end portions of the friction surface 10.
If the respective end portions of the disk brake pad 900 at the incoming and outgoing disk-rotor-rotation sides come into contract with the disk rotor, the surface contact pressure of these end portions is increased, whereby brake noise also increases. However, with the above disclosed configuration, the end portions of the disk brake pad 900 are chamfered so that contact of the end portions with the disk rotor reduces. Accordingly, it is possible to inhibit the generation of brake noise.
With the configuration shown in FIGS. 9A and 9B in which both end portions of the disk brake pad 900 are chamfered, the chamfering is performed such that a length of the friction surface 10 in the disk rotor circumferential direction Y1 remains substantially the same in the disk rotor rotation center direction Y2 (namely, the central axis K of the friction surface 10). When the disk brake pad 900 is configured in this manner, it is possible to realize improved brake noise performance.
However, as the friction surface 10 becomes worn along with usage of the disk brake pad 900, the percentage of the friction surface 10 that is accounted for by the chamfered portion J10 reduces, as shown by the dashed line in FIG. 9B. Thus, the noise reduction effectiveness of the chamfered portion J10 is gradually diminished, whereby brake noise reduction performance worsens.
Further, if wear of the disk brake pad 900 in the disk rotor rotation center direction Y2 (namely, the central axis K of the friction surface 10) is uneven, then brake noise reduction performance deteriorates even more.
FIG. 10 illustrates an explanation of why such uneven wear occurs. More specifically, FIG. 10 is a schematic view showing why uneven wear of the disk brake pad 900 occurs in the disk rotor rotation center direction Y2.
One of the causes of the uneven wear in the disk rotor rotation center direction Y2 of the friction surface 10 is a difference in work per unit area between a portion of the friction surface 10 that is closer to the disk rotor rotation center and a portion of the friction surface 10 that is farther from the disk rotor rotation center.
In other words, as shown in FIG. 10, a peripheral speed of the portion far from the disk rotor rotation center (an external periphery portion of the disk rotor) is faster than that of the portion close to the disk rotor rotation center (an inner peripheral portion of the disk rotor). Accordingly, the work of the external periphery portion of the disk rotor is higher.
Thus, the portion of the friction surface 10 of the disk brake pad 900 that comes into contact with external periphery portion of the disk rotor wears more than the portion of the friction surface that comes into contact with the inner periphery portion of the disk rotor. As a result, uneven wear of the disk brake pad 900 occurs, which causes the thickness of the disk brake pad 900 to become uneven. In other words, the portion of the disk brake pad 900 that is close to the disk rotor rotation center is thicker than the portion that is far from the disk rotor rotation center. In this way, uneven wear results from the work difference of the external and inner periphery portions of the disk rotor.
In addition, another cause of uneven wear is variation in pad surface contact pressure per unit area of the inside of the friction surface 10, which is caused by cylinder deformation that results from application of high hydraulic pressure.
FIG. 11 shows two of the disk brake pads 900, which have been assembled to a brake. The disc brake pads 900 are respectively positioned at an inside and an outside of a disk rotor 300.
For explanatory purposes, the disk brake pad 900 positioned to the inside of the disk rotor 300 will be called the “inner pad”, and the disk brake pad 900 at the outside of the disk rotor 300 will be called the “outer pad”.
In the brake shown, a piston 400 is moved by hydraulic pressure in the direction to the right of FIG. 11, whereby the inner pad 900 is pushed against the disk rotor 300. At the same time, a cylinder 500 is moved in the direction to the left of FIG. 11, whereby the outer pad 900 is pressed against the disk rotor 300.
FIG. 11 shows a state in which the outer pad 900 and the inner pad 900 have been pushed against the disk rotor 300 by the cylinder 500 and the piston 400, respectively. In this case, the above described uneven wear of the disk brake pads 900 has not occurred.
Note that, in FIG. 11, the reference numeral 600 is a mounting. This mounting 600 is a portion that is mounted to a vehicle, and is configured such that it acts as a braking torque receiving member. Further, a seal 700 is disposed between the piston 400 and the cylinder 500.
According to the brake mentioned above, when a fluid pressure, namely, the aforementioned hydraulic pressure, of the brake is high due to sudden braking or the like (in other words, the forces with which the cylinder 500 and the piston 400 push the disk brake pads 900 are strong), a pad surface contact pressure per unit area of the outer pad 900 in FIG. 11 becomes larger toward the external periphery portion side of the disk rotor 300 than the inner periphery portion side thereof.
This variation in pad surface contact pressure per unit area results from the fact that, amongst the area of the cylinder 500 that comes into contact with the outer pad 900, a region of the cylinder 500 that is located toward the inner periphery side of the disk rotor 300 is deformed more substantially than a region that is located toward the outer periphery side of the disk rotor 300. Accordingly, wear of the disk brake pad 900 occurs unevenly along the disk rotor rotation center direction Y2.
In this way, uneven wear of the disk brake pad 900 is caused by various factors such as (i) difference in the work of the outer periphery portion and the inner periphery portion of the disk rotor, and (ii) variation in the pad surface contact pressure of the inside of the friction surface 10 caused by low fluid pressure of the brake when braking gently.
Moreover, FIG. 12 shows the assembled brake of FIG. 11, in which uneven wear of the disk brake pads 900 has occurred.
When uneven wear of the disk brake pads 900 occurs as shown, the disk brake pads 900 are held less firmly by the cylinder 500 and the piston 400. Thus, when brake hydraulic pressure is low, the behavior of the disk brake pads 900 becomes unstable, which leads to the generation of brake noise.
To address these problems, as shown in FIGS. 13A and 13B, a configuration can be proposed in which the end portion of disk brake pad 900 at the incoming disk-rotor-rotation side and the end portion at the outgoing disk-rotor-rotation side are formed with respective chamfered portions J11, such that the friction surface 10 is formed with a fan-shape.
With this configuration, an external periphery portion of the fan-shaped friction surface 10 becomes the portion that is far from the disk rotor rotation center, and an inner periphery portion of the fan-shaped friction surface 10 becomes the portion that is close to the disk rotor rotation center.
Accordingly, the length of the friction surface 10 in the disk rotor circumferential direction Y1 becomes shorter in the disk rotor rotation center direction Y2 (namely, the central axis K of the friction surface 10).
Adoption of this fan-shaped friction surface 10 helps to promote both even distribution of work across the friction surface 10, and equal pad surface contact pressure per unit area in the disk rotor rotation center direction Y2 (namely, the central axis K of the friction surface 10). Accordingly, this configuration can be expected to reduce uneven wear of the disk brake pad 900.
However, when the disk brake pad 900 is configured with the fan-shaped friction surface 10 as shown in FIGS. 13A and 13B, it is necessary to make an angle θ (refer to FIG. 13A) large in order that the fan-shaped configuration is amply effective.
This angle θ is an angle formed between (i) the respective edges 11 and 12 of end portions of the friction surface 10 at the incoming and outgoing disk-rotor-rotation sides and (ii) the central axis K. Note that, in FIG. 13A, the angle θ is indicated as an angle formed between (i) the edges 11 and 12 of the friction surface 10 and (ii) respective axes K′ that are parallel to the central axis K of the friction surface 10. However, this angle is definitionally equivalent to the first definition of the angle θ above.
If the angle θ is made larger in this way, it is clearly apparent that the surface area of the entire friction surface 10 must be made smaller. This reduction in surface area leads the life of the disk brake pad 900 to become shorter due to an increase in wear thereof.